1. Field of the Invention
The present invention relates to an alloyed steel powder for metallurgy, which powder is suitable for use in the preparation of a sintered product of high density and high strength.
2. Description of the Prior Art
With development of alloyed steel powders there has been a demand for higher characteristics of sintered parts and higher density and higher strength are now required for alloyed steel powders to attain higher loading on sintered products. Especially improvement of density is effective on improvement of fatigue properties and toughness.
The strength of a sintered compact of an alloyed steel powder is generally improved by increasing the amount of alloy. In the conventional prealloying process, however, the compressibility of steel powder is deteriorated with increase in the amount of alloy, and according to a conventional powder metallurgy of a single pressing--single sintering type, it is now very difficult to attain both high density and high strength partly because of demand for a higher level of density and of strength. The demand for higher density may be satisfied by utilizing such a sinter forging process as is disclosed in Japanese Patent Laid-Open No. 44104/86. But this process involves many restrictions in point of the life of mold and the shape of product. On the other hand, according to a double pressing process wherein a single sintering operation is followed by repressing in a mold, there are few restrictions in point of the life of mold and the shape of product because the repressing is performed in a cold state like the single pressing process and thus the double pressing process is a more practical process.
However, in order to attain a higher density by such double pressing process, it is necessary to use a steel powder which affords as high density as possible in the first pressing and affords a still higher density in the second pressing after sintering which follows the first pressing. Such a steel powder is required to satisfy the following conditions:
(1) Should be superior in compressibility. PA1 (2) Generally, graphite powder is added at the time of sintering in order to enhance the strength of sintered steel. In this connection, in the first sintering which is performed usually at a lower temperature for a shorter time than in the second sintering, the steel powder should afford a sintered compact low in hardness and superior in recompressibility. PA1 (3) By a heat treatment which follows the second pressing and sintering operation, the sintered compact should become sufficiently high in strength in order to obtain a finally required strength. PA1 (i) prealloying, out of elements to be alloyed, an element less deteriorating the compressibility of steel powder, affording high hardenability in a small amount, being difficult to be reduced with hydrogen in the form of an oxide, and by diffusive adhesion, being difficult to be composite-alloyed while maintaining compressibility; PA1 (ii) on the other hand, partially diffusing and adhering an element to the surface of the prealloy obtained in the above (i) to effect composite-alloying, the said element being relatively easy to be reduced, easy to be composite-alloyed while maintaining compressibility by diffusive adhesion, being negative in affinity for carbon or forming a carbide positively to thereby suppress the diffusion of graphite into a sintered compact substrate during sintering at a low temperature, and capable of improving hardenability in a heat treatment after the second sintering. PA1 Cr: 0.1-5.0 wt. % PA1 V: 0.01-0.5 wt. % PA1 Nb: 0.001-0.01 wt. % PA1 B: 0.0001-0.01 wt. % PA1 Ni: 0.1-10.0 wt. % PA1 Cu: 0.1-10.0 wt. % PA1 Mo: 0.1-5.0 wt. % PA1 W: 0.1-5.0 wt. % PA1 (1) The uniformity of the sintered steel texture is improved, thus leading to improvement in strength and toughness. PA1 (2) When chromium is prealloyed into iron, its activity is reduced, so that the oxidation resistance is improved and it becomes possible to composite-alloying an oxide of an element which is more easily reducible than chromium. PA1 (3) Since chromium improves hardenability in a small amount thereof used, the compressibility of steel powder is scarcely deteriorated. PA1 (4) Chromium is less expensive and superior in economy as compared with nickel.
As alloyed steel powders for high strength there have been developed chromium-containing steel powders. For example, in Japanese Patent Laid-Open No. 164901/82 there is proposed a chromium-containing steel powder having enhanced compressibility and hardenability. In such alloyed steel powder, however, all the alloyed components, including chromium, are prealloyed, so where such alloy steel powder is applied to a double pressing process, graphite, which is added to improve the strength of the final sintered steel, easily dissolves into the steel powder as a sintered compact constituent at the time of the first temporary sintering, so that the steel powder hardens, thus leading to deteriorated recompressiblity.
In Japanese Patent Laid-Open No. 87202/83 there is proposed a method of diffusing and adhering chromium, in the form of a fine alloy powder with iron, to the steel powder surface. However, since an iron-chromium alloy usually contains a hard sigma phase, its direct use would cause wear of a mold at the time of powder molding. As means for solving this problem it may be effective to heat-treat the Fe-Cr alloy powder having a sigma phase into a soft alpha-phase compound for diffusion and adhesion to the steel powder surface. But there remains the problem that the steel powder manufacturing process becomes complicated.
Further, where chromium powder is to be diffused and adhered to the steel powder surface, since chromium has a strong affinity for oxygen, even if other alloy elements which are more easily reducible than chromium such as, for example, molybdenum and/or tungsten are to be diffused and adhered in the form of oxides to the steel powder surface together with chromium, chromium will be oxidized with the result that the function as the chromium alloy is no longer exhibited or the compressibility of the steel powder is deteriorated. Because of these problems, such method is not desirable.